Supercontinuum generation refers to the formation of broad continuous spectra by the propagation of a high power, relatively narrow spectral bandwidth laser radiation through a nonlinear medium. The continuum is produced via a number of nonlinear effects including self-phase modulation, Raman scattering and four-wave mixing. Initial experiments with supercontinuum generation used very high power lasers (mJ or μJ per pulse) with ultrashort pulses, typically <1 ps, focused into glass, sapphire and even water. The extremely high power of many megawatts was sufficient to create broad bandwidth spectra, typically spanning an octave or more (i.e. from a certain wavelength to twice that wavelength). However, the high power required meant that large, bulk laser systems were needed and often damaged the nonlinear media. In addition, the supercontinuum typically had an unstable and irregular transverse profile. The breakthrough was provided with the advent of specialised optical fibres providing high confinement within the core i.e. a high power density, which can propagate over long interaction lengths and therefore generate supercontinua at relatively lower laser powers and in a single transverse mode.
Optical fibres are commonly based on silica glass and in the spectral region beyond 1300 nm in which the material dispersion of silica glass is anomalous, optical fibres can be designed and fabricated to have a group velocity dispersion which is normal or anomalous, with a zero dispersion at any given wavelength over a wide range. It is not possible however, to move the zero dispersion wavelength of a conventional silica step-index, single-mode optical fibre to wavelengths less than 1270 nm, namely the zero dispersion wavelength of bulk silica.
The zero dispersion wavelength (ZDW) is an important parameter in the generation of supercontinua as, to-date, the widest spectra are produced when the pump wavelength, i.e. the laser wavelength that is coupled into the fibre, is close to the ZDW. In microstructured fibres or, more commonly, photonic crystal fibres (PCFs) however, it is possible to shift the ZDW of single mode fibres to much shorter wavelengths to thereby enable other laser sources having different wavelengths to be utilised as pump sources in the generation of supercontinua.
PCFs are most commonly formed from silica material and comprise a solid silica core region surrounded by a cladding region, which comprises an array of hexagonal-close-packed air filled holes in a silica background matrix. The air-silica cladding material creates an effective refractive index which is less than the refractive index of the core region and thus permits the guidance of light within the core by a variation of the traditional mechanism of total internal reflection.
PCFs are characterised largely by the diameter of the core, the ratio of the diameter (d) of the air holes in the cladding to their spacing, or pitch (Λ), and the ratio of the difference between the effective refractive index of the cladding region and core, to the refractive index of the core, often referred to as “delta” or Δ. PCFs can be manufactured to support a single confined mode (i.e. single mode fibre) or a plurality of confined modes (multimode fibre), respectively. More significantly, PCFs can also be manufactured to be endlessly single mode (ESM), such that ESM fibres can support only one mode for all wavelengths.
U.S. Pat. No. 6,097,870, hereinafter '870, and the corresponding scientific paper (Optics Letters, Vol. 25, No. 1, Jan. 1 2000—“Visible Continuum Generation in Air-Silica Microstructure Optical Fibres with Anomalous Dispersion at 800 nm”), discloses a microstructured optical fibre which is pumped at approximately 800 nm to generate optical supercontinuum in the range 390 nm-1600 nm. The fibre disclosed in '870 is inherently single mode at the pump wavelength which is the preference in the art given that single mode fibres have the best characteristics in terms of diffraction, transverse profile, stability and smallest spot size of the transmitted beam. Furthermore, single mode fibres do not exhibit modal dispersion, can carry more information than multimode fibres and are better at retaining the fidelity of each pulse over long distances. However, while the supercontinuum generated in the '870 patent spans from as low as 400 nm in the blue region to the infra-red region of the spectrum, the supercontinuum described in '870 can only be obtained using 800 nm pump wavelengths, and at this wavelength typical pump sources are unreliable and unfeasibly expensive for many applications.
There are two basic designs of PCF which have been used in the art for the generation of supercontinuum. One is the high-Δ fibre similar to that disclosed in the '870 patent. This type of fibre has the advantage of strong beam confinement giving a high nonlinear coefficient and a zero dispersion at short wavelengths.
The second type of fibre is the ESM fibre comprising relatively small diameter air holes in the cladding. This type of fibre can produce a zero dispersion wavelength around 1060 nm or 800 nm, but the nonlinear coefficient is not as strong. However, it has the advantage of being intrinsically single mode. The trend in the art is to use ESM fibres for pumping at wavelengths of 1060 nm.
It is an object of the present invention to provide a source of optical supercontinuum radiation for generating an optical supercontinuum having blue-enhanced spectral components.